When aspiring Manhattan actress Kristen was 27, she spent $5,000 to turn her God-given B cups to full C's. She chose saline implants with a slicone core, a product that—along with all silicone falsies—was pulled off the market in 1991. She knew to massage her breasts regularly to try to prevent too much scar tissue from forming around the implant, the natural result of any foreign object being placed inside the body. She knew to remain on the lookout for sudden deflation, which would have signified a rupture. She didn't realize that, like the rest of her body, her brand-new breasts would wear out over time.

Eighteen years later: "From the front, they still looked kind of good," Kristen says. "But from the side, they looked like a lot of my friends' who'd breast-fed. They didn't have any boost to them. They looked completely real! I went in for a mammogram and the technician said, `I thought you said you have implants?' When I got them done, I didn't know they had a shelf life. I don't recall anyone telling me that."

When Kristen had her breasts done in 1990, a boob job was still a major event. Not anymore. Breasts, like bad teeth or unruly eyebrows, have become something to "fix." A new set still costs only around $5,000, the same price paid by Kristen when Bush Sr. was in office, some doctors need less than an hour to insert the implants, some workaholics take just a five-day weekend away from the office to recuperate. Breast augmentation was the most popular cosmetic surgery for women in 2007 (closely followed by lipo), according to the American Society for Aesthetic Plastic Surgery (ASAPS). But many of this next generation of implantees, just like Kristen, assume their new breasts will stay perfect and perky even as the rest of their bodies inches toward the floor.

"Twenty-five years ago, women weren't told that implants weren't permanent," says Foad Nahai, MD, a plastic surgeon in Atlanta and the current president of ASAPS. "But patients I see now are still surprised. The first thing I tell any new patients is, `Please understand, these won't be your last pair.'"

Officially, implants themselves don't have set expiration dates. "They last as long as they last. It could be six months, 10 years, or 20 years," says David Rapaport, MD, a New York City plastic surgeon.

On average, though, according to Lawrence Reed, MD, a plastic surgeon with a practice right off of Park Avenue, between 20 and 30 percent of patients will need another procedure within 10 years. Which means that a college freshman with a Happy High School Graduation! boob job (the majority of breast augmentation patients are between the ages of 19 and 34) can expect at least three revisions if she lives to collect Social Security.

According to the FDA, you can blame it on the implants only about 10 percent of the time. A 2004 FDA Institute of Medicine report estimated that one out of 10 of both saline and silicone models rupture by the five-year mark. But more often, the complications that land a woman back in the OR have less to do with the implant itself than with the way her individual body ages, the elasticity and thickness of her skin, and her propensity for scar tissue. A whopping 51,000 of American women—approximately 13 percent of the 399,440 who got breast enhancements in 2007—can expect sometime in the next four years to develop capsular contracture, one of the most common side effects of augmentation, in which the body's protective layer of scar tissue tightens around the implant. When Kerri, 38, a former swimsuit and lingerie model in Chicago, got her silicone falsies at age 20, she "didn't ask a lot of questions," she admits. Six months later, due to contracture, her breasts "felt like two baseballs," she says.

Kerri's case developed quickly, but contracture can happen at any time, "starting from day one to year 40," says Laurie A. Casas, MD, an associate professor of surgery at Northwestern University's Feinberg School of Medicine in Chicago. "I had a lady, 78 years old. Her implants were put in in 1969. Thirty-eight years later, all of a sudden one was soft and one was a rock. It bothered her when she hugged her grandchildren."

No one knows exactly what causes contracture. Some doctors suspect low-grade bacterial contamination, residual blood around the implant, or even inflammation in the capsule wall. Accolate, an asthma drug that inhibits inflammatory response, is believed by certain physicians to prevent hardening by as much as 30 percent when taken for three to six months postop, but the drug can also cause liver damage.

Casas studied the drug's effect on patients with saline implants positioned under the muscle and reported statistically significant reversal in early capsular contracture. Still, plenty of her colleagues, such as David Hidalgo, MD, a New York City surgeon, remain unconvinced. "It's a little bit of mythology right now," he says. "I wouldn't want to trade a hard breast for liver failure."

Kerri didn't do anything until she developed yet another problem at age 32. The mother of four noticed her breasts had what is called a "double bubble." Because of stiffness from the contracture, her implants sat up high on her rib cage. Her skin and natural breast tissue had slid down and pooled below the implants. On profile, it looked as if she had two sets of breasts, one higher and one lower. To fix this, the scar tissue had to be scraped out entirely and a new pocket created, this time under the muscle. Rather than trimming away the excess skin, Kerri chose slightly larger implants to fill it out. When the originals were removed, they were intact and perfectly soft. "It was my scar tissue that was making them solid," she notes.

A change in body weight can trigger the need for surgical revision. Kristen, the actress, became unhappy with her implants once she gained and then promptly lost 40 pounds after having her daughter. The repeated expansion and contraction—her 34C's swelled to 40E's, then fluctuated daily from breast-feeding—thinned out her breast tissue the same way it would have if she had been implant-free. Says Reed, the plastic surgeon who treated her, "Implants are not a protector. They're there for the ride."

Significant weight loss, especially if a woman has saline implants inserted above the pectoral muscle, may cause the edges of the pouch to show through her skin. Michelle, an executive at a consulting company in Riverdale, New York, had two kids before she opted for saline implants at 27. "I didn't even know if I could fit into a bra," she says of her pre-implant chest. "I just had hanging skin. I guess I could have rolled them up into an A cup." Her new boobs "felt huge in a good way," and for seven years she was very happy with them. Then she took up competitive bodybuilding.

Michelle was already thin—her total body fat hovered around 18 percent—but thanks to her increased muscle mass, that percentage dropped to 13. "I started noticing I had ripples in my breasts like a string of cellulite," she says.

These waves aren't "a product defect," Casas says. "If you put an implant on a desk and let it sit there, the edges of the product form little hills and valleys. It's just more palpable when the tissue is very thin."

Casas suggests switching to silicone, which Michelle ultimately did. "But if you get silicone and you still have rippling because your skin is so thin," Casas says, "your choice is to accept it or have your implants taken out."

"The biggest issue I have is managing expectations," says Sean Simon, MD, a plastic surgeon in Miami. "When you put implants in, they sit up high. But if you want that forever, you have to go live on the moon. Here on Earth, you're subject to the laws of gravity like everyone else." He tries to get patients to consider the larger picture, such as whether or not the size of their new chests will fit their lifestyle down the road. (Should a golfing grandma have double-D's?) When patients ask for implants that are "inappropriately large for their dimensions," Simon says, "I tell them that there's a bigger chance they'll need another operation sooner, whether it's to go even larger to fill out the stretched, sagging skin, reposition them, take them out, lift them—something. That's a fact."

Even with all the things that can go wrong with fake breasts as they age, and even if women have multiple, sometimes painful, procedures to fix them, most choose to replace their implants rather than simply remove them. Michelle had two more surgeries after her initial correction, for a total of six over 13 years. She suffered through contracture, double bubbling, and bouts of nerve pain, and now has scars around her nipples and underneath her breasts from four to eight o'clock. Still, life au naturel was never an option. "My friends were begging me not to do it. They were scared. But when I lost all the weight, I had nothing left, and I don't want to have nothing. It makes me feel like a woman to have breasts," she says. "Plus, I kind of like that going-under feeling. Does that make me a plastic surgery junkie?"